The present invention relates to the field of calendering machines, and more particularly to devices for controlling the diameter of the rolls used in calendering machines.
Pressing a material between two calender rolls can change the physical characteristics of the material. For example, calendering paper changes it density, thickness and surface features. Thus, the calendering process is frequently used in the manufacture of paper and other sheet materials to control the characteristics of the sheet.
A common problem associated with calendering is the uneven thickness of the calendered sheet material, or "web". Localized variations in a variety of parameters, including the moisture content of the web, creates variations in the spacing or "nip" formed between cooperating rolls. Variations in the nip across the width of a pair of calender rolls produces a web having a non-uniform thickness. However, if the calender rolls are made of a material that expands and contracts with changes in temperature, one may control the diameter of at least one of the rolls along its axis by varying the temperature of selected cylindrical sections of the roll. The thermal expansion and contraction of the roll allows the calender roll operator to obtain a more uniform web thickness.
A number of previously known devices have heated and cooled the roll sections with jets of hot and cold air. These devices blow jets of hot air from a hot air supply plenum against sections of the calender roll which are producing web that is too thick. Each hot air jet heats the section of roll against which it is directed, thereby causing the heated section to thermally expand. As the heated section expands, the nip between the heated roll section and the adjacent cooperating roll decreases, thus applying greater pressure to the web. The increased pressure, of course, decreases the thickness of the web pressed by the heated roll section. Alternatively, when these devices blow jets of cold air, from a separate cold air supply plenum, against the selected cylindrical sections of the calender roll, the cooled sections of the roll contract. This decreases the local roll diameter and therefore increases the thickness of the calendered web which is pressed by the cooled roll sections.
In these previously known devices, nozzles communicating with the interior of each hot and cold air plenum are used to direct jets of air against the calender roll. The nozzles are disposed along the hot and cold air supply plenums at intervals corresponding to the adjacent sections of the calender roll whose local diameters are to be controlled. Examples of such devices are shown in U.S. Pat. No. 2,981,175 to Goyette, U.S. Pat. No. 3,177,799 to Justice and U.S. Pat. No. 3,770,578 to Spurrell. These previously known devices use valves to control the flow of air through each nozzle. Since separate plenums provide the hot air and cold air, these devices require two valves and two nozzles to control the diameter of each section of the calender roll. Alternatively, a dual control mechanism may be used to mix the hot and cold air from the two plenums and then release the air through a single nozzle. In either configuration, this redundancy can increase the cost of these devices
Another problem experienced with previously known calender roll controllers results from the fact that accurate control of the roll diameter requires precise metering of the air jets. Therefore, the valve control mechanisms generally should not exhibit hysteresis effects so that they can obtain repeatable settings regardless of whether the valve is being opened or closed. Furthermore, these control mechanisms usually must be capable of operating at both high and low temperatures. However, even when the valves work properly and the control mechanisms accurately control the size of the valve orifices, the rate at which air is released through the nozzles is often variable because the air pressure in each plenum depends upon both the number of valves open at one time and the volume of air released through each nozzle. Thus, the flow of air through the nozzles in these devices can be difficult to control.
Many of these previously known devices are subject to still other limitations and inefficiencies. For example, the nip control range is a function of the maximum and minimum temperatures of the air jets. However, the hot air in the hot air plenum is typically heated by waste steam from the power plant for the calender roll facility. Steam supplied by such a power plant usually has a maximum temperature of about 350.degree. F., and inefficiencies in the heat exchange process further limit the maximum temperature of such steam heated air to about 325.degree. F. Furthermore, to maintain the air temperature in the hot air plenum at 325.degree. F., hot air must be continuously supplied to the hot air plenum, even when hot air is not being released through the nozzles. If hot air is not continuously supplied to the hot air plenum, the stagnant air in the plenum may cool to ambient temperature. Then, when a jet of hot air is required to increase the diameter of a section of the calender roll, the cooled stagnant air must first be purged from the plenum. This increases the response time of the device.
My previously filed copending applications, Ser. Nos. 694,855 and 695,438, are directed to calender roll controllers which eliminate many of the disadvantages of these previously known roll controllers. The devices described in these applications provide a constant flow of air from a single plenum through a plurality of nozzles. Each nozzle directs a jet of air from the plenum toward an opposing section of the roll whose diameter is to be controlled. However, instead of using valves to control the flow of air from hot and cold air plenums, as was done in previously known devices, the devices described in my previously filed applications use individually controllable electric air heaters to selectively control the temperature of the air jets. The heating and cooling of the calender roll sections by the temperature controlled air jets controls the diameter of the various roll sections by thermal expansion and contraction.
One particular form of my previous inventions comprises a single elongated plenum positioned lengthwise alongside a calender roll. A plurality of holes are formed at intervals in the wall of the plenum which faces the curved surface of the roll. Tubes are disposed inside the plenum so that the front end of each tube is aligned with one of the holes in the plenum wall. The plenum may be pressurized with ambient room temperature air so that the tubes inside the plenum direct jets of air from the plenum at opposing cylindrical sections of the rotating roll. Heating elements, such as coiled electrically resistive nichrome wire, are disposed inside each tube. Therefore, when a particular heating element within one of the tubes is energized, the air escaping through that particular tube is heated by contact with the energized heating element as the air flows along the length of the tube. Since the device does not have individual valves to control the flow of air through each tube, the rate with which air is emitted by each tube remains substantially constant. Only the temperature of the air jets change as more or less power is supplied to each of the heating elements within the tubes.
Since the calender roll controllers of my previously filed applications require only one plenum and can operate without any air flow control valves, these controllers have a relatively low initial cost. Additionally, because they use electric heating elements rather than steam heaters, the hotter air jet temperatures obtainable with these electric heaters can produce approximately two to five times the nip control range on a typical 12-14 inch diameter 190.degree. F. calender roll.